This invention relates to line interface devices or data access arrangements (DAA) for communications and, more particularly, to a line interface device which provides galvanic isolation between data communications equipment and a digital subscriber line (DSL).
Wide area networks (WANs) are used to transfer data over long distances and large geographic areas. These networks utilized communications equipment, typically modems, to convert the digital data to a format compatible with the appropriate transmission standard used by the WAN service. For example, analog modems convert digital data to a data stream encoded in an audio carrier signal which can be transmitted and received over ordinary telephone lines in accordance with the ITU-T V.34 and V.90 standards. Other types of WAN services, such as Digital Subscriber Line (DSL) and Broadband Cable services utilize xDSL and Cable modems, respectively, to transfer data over the respective WANs. Digital Subscriber Line technology currently enables users to obtain over 6 Mbps data transfers from the Central Telephone Office (CO) to the user over ordinary existing copper telephone line pairs. There currently exist several different types of DSL services, and accordingly as used herein, the term "xDSL" is intended to include all the different types of DSL services including, for example, Asymmetrical DSL (ADSL).
The latter provides a data rate which is different depending on which direction the data is being transmitted. Currently, ADSL modems typically support a rate of about 6 Mbps downstream and a rate of about 640 Kbps upstream (where "upstream" is defined as data from the user to the CO, and "downstream" is defined as data from the CO to the user). The asymmetric nature of the technology makes it suitable for Internet access since the majority of data in an Internet connection is transmitted downstream to the user, with mouse key clicks being the majority of data sent upstream.
In general, WAN service providers, for example, the local telephone company, typically require a certain level of electrical or galvanic isolation between the communications equipment and the WAN medium. Isolation is required in order to provide a level of fault tolerance in the WAN service provider's network. It is desirable that if a piece of communications equipment connected to the WAN fails, it does not damage or interfere with the communications medium or other devices connected to the communications medium.
Thus, where an xDSL modem is used at the user location, there exists the same interface device requirements of the standard analog modem such as: signal coupling to the telephone line, isolation of the modem from the telephone line; over voltage and surge protection; and a 2-4 wire hybrid.
Typically, the industry standards require 1500 volts of galvanic isolation between the line and modem circuitry. In current xDSL equipment, isolation is provided by a coupling transformer connected between the modem and the telephone line. While the coupling transformer can provide the necessary line isolation, coupling transformers are typically too large and bulky (in the order of 0.5 cubic inches) to be used in devices requiring a small form factor, such as the credit card sized PC Card which may be used with a notebook computer.
Optical isolation interface devices have been proposed for interfacing analog modems and an analog wide area network because, among other things, they take up less space and are amenable to IC implementation. Until the present invention, however, optical isolation interface devices have been considered impractical for interfacing xDSL modems with an xDSL line. More specifically, optical interface devices usually include drivers on the "line" side of the interface device, requiring a certain amount of power from the line side of the interface device to operate these drivers. Implementing the arrangement with an analog modem and analog line is relatively straight forward since power is available, and therefore can be derived from the telephone line to operate the line drivers of the interface device so that isolation is preserved. Unfortunately, with xDSL, there is either no power available from the line or the power required by the line drivers of an optical isolation interface device is substantially greater than can be derived from the line.
To facilitate a better understanding of the foregoing, reference is made to FIG. 1 which shows a typical unit of data communications equipment (DCE), and more specifically a modem 100, which is adapted for using industry standard data communications protocols to transfer data over a wide area communications network (WAN). The modem 100 includes a modem controller 120 which is adapted for controlling substantially all of the operations of the modem 100. The modem controller 120 is adapted, via a communications interface device 110, for receiving data from and transmitting data to the local DTE (data terminal equipment) (not shown). The data to be transmitted to the remote DTE is formatted in accordance with the appropriate data communications protocol and transferred, via an internal bus 130, to the data pump 140. Similarly, data from the remote DTE is extracted from the protocol data units (PDUs) received from the data pump by modem controller 120 and transferred to the local DTE. The data pump 140 transfers data to and receives data from the line interface device 200 via internal bus 150. The line interface device 200, which is sometimes referred to as a "data access arrangement" or DAA, is connected to the telephone line 12 and transfers data to and from the communications network 10.
In the case of xDSL the arrangement is similar to existing analog modem front ends with some fundamental differences. While the DSL modem can operate on existing telephone lines, it is required that equipment changes at the CO be made to accommodate this new technology. Some of the significant differences related to the line interface device are:
Analog Modem DSL Modem Bandwidth 300 Hz-3400 Hz 300 Hz-1.1 MHz Power level 0 dBm Max +26 dBm Max Line Impedance 600.OMEGA. 130.OMEGA.
Most telephone lines made of copper wire can support 1.1 Mhz when they are unloaded. Loaded lines are lines which have large inductors placed in series at certain locations along the line to improve voice response. Unfortunately, these inductors form a low pass filter which hampers high speed data connections. These inductors must be removed in most cases for xDSL communications to achieve full speed of data throughput. Fortunately, most lines do not have loading inductors and the ones that do are usually restricted to rural lines over long distances.
Until recently, the high power required to support ADSL with Discrete Multitone Modulation (DMT) is +26 dBm. In order to achieve this power level, 15V power supplies are required for the line drivers and, in addition, the coupling transformer is usually a step up type as opposed to a 1:1 ratio. More recently, the power requirements have been somewhat relaxed so that 10V power supplies are required.
It is clear that two major drawbacks to the existing design are that designers must provide dual 15V or 10V power supplies in addition to the requisite +5V usually required for system operation. Further, the transformer size is relatively large making the overall interface device cumbersome.